Most modern radio systems have 50-ohm impedance RF terminations. This refers to the transmitter output, the transmission lines, the receiver input, the antennas, etc. In any radio system, when all of the component system equipment has the same impedance, the Maximum Power Transfer Theorem is satisfied. The theorem simply states that the maximum possible power transfer between source and load is obtained when the source impedance and load impedance are equal. This is the basis of the 50 ohm RF transmission system which is the international standard. If the system has a 50-ohm impedance, then the system operates at near-perfect efficiency.
The measure of the “quality” of the impedance match, and hence the power transfer efficiency, is called Standing Wave Ratio (“SWR”). SWR is the ratio of impedances, expressed as SWR=ZSOURCE/ZLOAD. (If ZLOAD>ZSOURCE, then SWR=ZLOAD/ZSOURCE). An SWR ratio of 1:1 is perfect with 100% efficiency of power transfer. A system design attempts to get as close to a 1:1 SWR as possible. That said, a modern transmitter operates well when the load SWR is on the order of 1.3:1 to 1.5:1 or less.
An antenna coupler or line flattener can be used to match the 50-ohm output impedance of a transmitter to the actual impedance of an antenna by transforming the antenna impedance to 50 ohms via a passive electronic circuit. A line flattener is similar to an antenna coupler but there are important differences and specific advantages of a line flattener versus an antenna coupler. An antenna coupler has an impedance transformation range of 100:1 or more, and can transform very high standing wave ratio (“SWR”) values of a resonant antenna (i.e., whip) to within the range of a transmitter's SWR capability. A line flattener can be viewed as a simplified antenna coupler with an impedance transformation range of 4:1 (typical), and transforms moderate SWR values of a wideband antenna to within the range of a transmitter's SWR capability. In this regard, the operating principles of an antenna coupler and a line flattener are similar, except that the transformation range, i.e., SWR transformation limits, differs between the two.
Prior art line flatteners have resulted in designs that are coarse antenna couplers, using discriminator tuning (that measures the actual complex radio frequency impedance of the antenna using a complex and expensive electronic subsystem) to give a coarse impedance match to present a low SWR at the selected frequency. This included all the complexity and cost burden of a classical antenna coupler.
Accordingly, there is a need for a more efficient and less complex line flattener with the advantages of an antenna coupler.